1. Field of the Invention
This invention relates to a method for the frequency stabilization of a laser oscillated from an internal mirror type helium-neon laser device, which has a structure with a laser capillary disposed in a laser tube, and having an oscillation wavelength of 543 nm.
2. Description of the Related Art:
As an oscillation wavelength of an internal mirror type helium-neon laser, a wavelength of 633 nm (red) has been known to date.
As means for stabilizing the oscillation frequency of the internal mirror type helium-neon laser having an oscillation wavelength of 633 nm (hereinafter called "633 nm He-Ne laser"), have already been established many controlling techniques, for example, two-mode method, Lamb dip method, iodine-absorption cell method, longitudinal Zeeman method, transverse Zeeman method, magnetic modulation method, etc.
On the contrary, an internal mirror type helium-neon laser having an oscillation wavelength of 543 nm (green) (hereinafter may called "543 nm He-Ne laser") is a relatively new laser the first oscillation of which was reported by Perry in 1970 [D. L. Perry, IEEE J. Quantum Electron, QE-7, 102 (1971)].
Since the oscillation wavelength of this 543 nm He-Ne laser is shorter than that of the 633 nm He-NE laser, it can be expected that its application to precision instrumentation will results in improved accuracy of the instrumentation. On the contrary, the 543 nm He-Ne laser however is extremely small as about 1/15-1/17 in gain of transition (3s.sub.2 .fwdarw.2p.sub.10) compared to the 633 nm He-NE laser. Its practical use has hence been difficult for a long time.
Recently, it has however been possible to enhance the oscillation output of the 543 nm He-Ne laser to the practical extent with a cavity length of 40 cm or so owing to the improved performance of the laser mirrors used therein.
In 1987, experimental results on frequency stabilization conducted by making use of a commercial 543 nm He-Ne laser were reported by T. Fellman et al. [Applied Optics, 126 (14), 2705 (1987)].
The existence of so-called polarization flipping, in which the polarization directions of axial modes polarizing orthogonally and linearly in the vicinity of the region where the axial modes become symmetrical configuration for the center of gain suddenly interchange by 90.degree., has been definitely shown by this report.
Accordingly, if the frequency stabilization of the 543 nm He-Ne laser is performed, for example, by using the two-mode method, it is necessary to avoid the region at which the above-mentioned polarization flipping occurs (hereinafter called "polarization flipping region").
The two-mode method mentioned above is a method making use of properties that polarizations in adjacent axial modes always become orthogonal and linear, and attempting the frequency stabilization of a laser by separating the polarizations in the adjacent axial modes by means such as a polarized beam splitter (PBS) or the like and then using their intensity difference or intensity ratio as an error signal for controlling the oscillation frequency of the laser.
The cavity lengths of laser devices commercially available at present are however adjusted to the extent of about 40 cm in order to enhance their output to practical levels. Therefore, the 543 nm He-Ne laser usually is found to oscillate in a range of 3-4 axial modes as illustrated in FIG. 1. Because a laser tube expands with heat and its cavity length is hence elongated when electrical input power is turned on to the laser tube, the axial modes change repeatedly like (a).fwdarw.(b).fwdarw.(c).fwdarw.(d).fwdarw.(a) . . . in FIGS. 1(a)-1(d). In FIGS. 1(a)-1(d) C.sub.1 and C.sub.2 are characteristicc polarization directions of the laser tube. C.sub.1 is the weak direction while C.sub.2 is the strong direction, both, in light intensity.
However, the 543 nm He-Ne laser is greatly different from the 633 nm He-Ne laser, which has already been forward in putting it to practical use, in that:
(1) adjacent axial modes do not necessarily polarize orthogonally, but the group of parallel polarizations is always present; and
(2) in the case of an axial mode configuration such that the gain competition between the modes becomes strong, namely, in the case of FIG. 1(b) or FIG. 1(d), the polarization flipping, wherein the polarization directions of each mode suddenly interchanges by 90.degree., occurs.
Accordingly, when the frequency stabilization of the 543 nm He-Ne laser is performed, for example, by using the two-mode method as is, there are encountered the following problems:
(1) the region capable of stabilizing frequency is limited; and
(2) since the frequency stabilization can be effected only in the mode configuration and polarization state illustrated in FIG. 1(a) or FIG. 1(c), one polarized component comes to contain 2 frequency components in which their frequency difference is a axial mode spacing, whereby the laser becomes disadvantageous when it is used as a light source for a polarization interfero-water.